Apparatus for Separating Oxygenate Modifier from Oligomerization Effluent by Water Wash

ABSTRACT

It has now been disclosed that by directing the effluent from an oligomerization reactor to a water wash column to remove alcohol modifier from the hydrocarbon stream before sending it to a fractionation column offers flexibility in providing a bottoms product of a desired vapor pressure without increasing the concentration of alcohol modifier in the overhead stream beyond alcohol concentration limits.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.10/125,630 filed Apr. 18, 2002, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a process and apparatus for separatingoxygenate modifiers from product oligomerized hydrocarbons.Specifically, alcoholic modifiers are separated from oligomerizedhydrocarbons by a water wash column.

BACKGROUND OF THE INVENTION

Processes for the oligomerization of light olefins to produce C₈ olefinoligomers are known. Oligomerization processes have been long employedto produce high quality motor fuel from C₄ olefins. Such oligomerizationprocesses are also referred to as catalytic condensation andpolymerization with the resulting motor fuel often referred to aspolymer gasoline. Methods have always been sought to improve the octanenumber of the gasoline boiling range oligomerization products. Indirectalkylation is a noteworthy C₄ olefin dimerization process.

In one form of the indirect alkylation process, an ionic exchange resincatalyst oligomerizes light olefins to produce oligomers such as C₈olefins. In such processes, the oligomerization zone can be preceded bya dehydrogenation zone to convert paraffinic feed into olefinic feedand/or succeeded by a hydrogenation zone to convert heavy oligomericolefins into heavy alkanes that can be blended with gasoline stock.

U.S. Pat. No. 4,313,016 discloses a heat exchanged oligomerizationreactor that contains a cationic exchange resin catalyst. C₄ olefinscontacted with the resin catalyst oligomerize to C₄ oligomers. Water ormethanol may be present in small amounts insufficient to form anentrained second phase to serve as a catalyst modifier.

Modern oligomerization processes often include an oxygenate such astert-butyl alcohol (TBA) and/or sec-butyl alcohol (SBA) in the feed formodifying the catalyst to maintain desired product selectivity. Themodifier does not participate in the reaction. References disclosingresin catalyzed oligomerization in the presence of an oxygenate modifierinclude U.S. Pat. No. 5,877,372 and EP 994 088 A1. TBA and SBA havebecome the resin catalyst modifier of preference.

More recently, higher quantities of alcohol modifier have been used inresin catalyzed oligomerizations. Consequently, removing the alcoholmodifier from the hydrocarbon oligomerization product stream has becomemore important. In such oligomerization processes, it is typicallynecessary to separate unreacted light olefins from the product heavyoligomers in the effluent from the oligomerization zone. Separation isconventionally performed in a distillation column typically followingthe oligomerization zone. The lighter components comprising primarilyunreacted C₄− olefins and compounds that were present in the feed streamexit from the overhead of the distillation column. The heaviercomponents comprising primarily heavy oligomers such as C₅+ olefins andcompounds exit out the bottoms of the distillation column. If thedistillation column is operated to send all of the C₅+ materialcontained in the oligomerization effluent to the bottoms, most of thealcohol modifier would exit with the bottoms product and only a smallamount of alcohol modifier would exit in the overhead stream. A waterwash column was designed to treat the bottoms product and recover thealcohol modifier before the C₅+ stream proceeded to product storage orfurther treatment.

Reid vapor pressure is a standard unit used in governmentalspecifications regarding gasoline product vapor pressures. To meetincreasingly tight governmental specifications, the distillation columnmust be operated so as to control the vapor pressure of the bottomsproduct. In this case, some of the C₅ compounds are diverted from thebottoms to the overhead product. Consequently, a portion of the alcoholmodifier will azeotrope with the C₅ material and both the overhead andbottoms stream will contain alcohol modifier. As such, alcohol modifiermust be removed from both the overhead and the bottoms product streams.

U.S. Pat. No. 4,956,513 discloses an oligomerization process that uses ahomogeneous boron trifluoride catalyst with a promoter such as normalbutanol. After the oligomerization, the boron trifluoride catalyst isextracted from the reactor effluent by water washing. The water extractcontaining the major part of the boron trifluoride catalyst is thendistilled to remove the water in the promoter.

U.S. Pat. No. 5,146,032 discloses reacting C₃+ olefins with methanolover a ZSM-5 catalyst to produce a range of hydrocarbons including anolefinic gasoline stream. The unreacted methanol and water present areseparated by cooling, phase separation and, in some cases, by waterwashing of the hydrocarbon effluent leaving the reactor. They are thenled to a methanol-water separator such as a distillation tower.

An object of the present invention is to fractionate oligomerizationeffluent to provide a heavy oligomer bottoms product with a desiredvapor pressure and a light olefin overhead stream with a sufficientlylow concentration of alcohol modifier.

An additional object of the present invention is to fractionateoligomerization effluent without having to utilize a separate water washcolumn on both the heavy oligomer bottoms product and the light olefinoverhead streams.

SUMMARY OF THE INVENTION

It has now been discovered that by directing an effluent from anoligomerization reactor to a water wash column to remove alcoholmodifier from the hydrocarbon stream before sending it to afractionation column offers flexibility in providing a bottoms productof a desired vapor pressure without increasing the concentration ofalcohol modifier in the overhead stream beyond alcohol concentrationlimits. The present invention recognizes the heretofore unknown problemthat if greater quantities of C₅ hydrocarbons are allowed to go up inthe overhead to adjust the vapor pressure of the bottoms product of thefractionation column, too much alcohol modifier goes with the C₅hydrocarbons out with the overhead stream. Hence, it was difficult toprovide a bottoms product that comprises olefinic gasoline that can meetthe vapor pressure specification and, at the same time, provide anoverhead product comprising mainly unreacted C₄ hydrocarbons that canmeet the alcohol specification. The present invention solves thediscovered problem by inserting a water wash column prior to thefractionation column instead of having to separately water wash each ofthe overhead stream and the bottoms stream from the fractionationcolumn. The water wash column adequately removes all but tracequantities of alcohol modifier from the hydrocarbon stream which canthen be fractionated in the distillation column at the desired cutbetween C₄ hydrocarbons and C₅ hydrocarbons to meet specifications inboth the overhead and bottoms streams.

Accordingly, in one embodiment, the present invention relates to aprocess for oligomerizing light olefins and recovering heavy olefinscomprising feeding a reactant stream of light olefins and a non-reactantstream of alcohol modifier with at least three carbons to a reactorcontaining a solid resin catalyst; catalytically oligomerizing the lightolefins to heavy olefins in the reactor; passing a reactor effluentstream comprising heavy olefins, unreacted light olefins and alcoholmodifier to a water wash column; washing the reactor effluent streamwith water in the water wash column; withdrawing a wash effluenthydrocarbon stream comprising heavy olefins and unreacted light olefinsfrom the water wash column; withdrawing an extract stream comprisingwater and alcohol modifier from the water wash column; passing the washeffluent hydrocarbon stream to a separation column; separating the lightolefins from the heavy olefins; and recovering heavy olefins from theseparation column.

In another embodiment, the present invention relates to an apparatus foroligomerizing light olefins and recovering heavy olefins comprising anoligomerization reactor vessel including a feed inlet for deliveringlight olefins to the reactor vessel and a product outlet for removinglight olefins and heavy olefins from the reactor vessel; a water washcolumn including a hydrocarbon inlet in communication with the productoutlet of the reactor vessel, a water inlet near a top thereof, a wateroutlet near a bottom thereof and a hydrocarbon outlet near a topthereof; and a separation column including a hydrocarbon feed inlet incommunication with the hydrocarbon outlet of the water wash column, alight olefin outlet near a top thereof and a heavy olefin outlet near abottom thereof.

In a further embodiment, the present invention relates to an apparatusfor oligomerizing light olefins and recovering heavy olefins comprisingan oligomerization reactor vessel including a feed inlet for deliveringlight olefins to the reactor vessel and a product outlet for removinglight olefins and heavy olefins from the reactor vessel; a water washcolumn including a hydrocarbon inlet in communication with the productoutlet of the reactor vessel, a water inlet near a top thereof, a wateroutlet near a bottom thereof and a hydrocarbon outlet near a topthereof; a separation column including a hydrocarbon feed inlet incommunication with the hydrocarbon outlet of the water wash column, alight olefin outlet near a top thereof and a heavy olefin outlet near abottom thereof; and a stripper column having an inlet in communicationwith the water outlet of the water wash column, the stripper columnhaving a water outlet in communication with the water inlet of the waterwash column for recycling water from the stripper column to the waterwash column.

Additional objects, embodiments and details of this invention can beobtained from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates a flow scheme for the distillative separation ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for directing effluent from anoligomerization reactor containing unreacted light hydrocarbons, heavyproduct hydrocarbons and oxygenate modifier to a water wash column toremove the oxygenate modifier from the hydrocarbons. The oxygenatemodifier is preferably an alcohol. The water washed hydrocarbon free ofall but trace quantities of alcohol modifier are then fractionated in adistillation column. Hence, the split between light hydrocarbons andheavy hydrocarbons in the distillation column can be adjusted to obtaindesired properties without having to water wash both the overhead andbottoms streams from the fractionation column separately. The extractedwater/modifier stream is then stripped to separate the water and thealcohol modifier. Both separated components are then recycled back tothe process.

Preferred catalyst for the oligomerization reaction can generally bedescribed as protonic acids. The preferred acids will generally have aHammett acidity function of −4.0 or less. Examples of catalysts fallinginto this category include phosphoric acid catalysts. Solid phosphoricacid catalyst has a Hammett acidity function of approximately −5.0 orlower. A particularly preferred catalyst is a sulfonic acid ion-exchangeresin catalyst. This resin catalyst comprises sulfonic acid groups andcan be prepared by polymerizing or copolymerizing aromatic vinylcompounds followed by sulfonating. Examples of aromatic vinyl compoundsinclude the following: styrene, vinyl toluene, vinyl naphthalene, vinylethylbenzene, methyl styrene, vinyl chlorobenzene and vinyl xylene. Anacidic ion-exchange resin contains typically approximately 1.3 to 2.0sulfonic acid groups per aromatic group. Preferred resins are thosebased on copolymers of aromatic monovinyl compounds and aromaticpolyvinyl compounds and in particular divinyl compounds in which theconcentration of polyvinyl benzene is approximately 1 to 20 wt-% of thecopolymer. The particle size of the ion-exchange resin is preferablyapproximately 0.15 to 1 mm. Furthermore, perfluorosulfonic acid resinsconsisting of copolymers of sulphonylfluorovinyl ethyl and fluorocarboncompounds can be used. Various suitable ion-exchange resins arecommercially available under the name, for example, Amberlyst 15. Theconcentration of the catalyst is typically 0.01 to 20% of the mixture itis catalyzing and preferably 0.1 to 10% of the weight thereof.

The feed for the oligomerization reactor will typically be a C₄ cut froma debutanizing distillation column that follows a fluidized catalyticcracking (FCC) unit. The feed will typically comprise C₃ to C₅ aliphaticolefins. A non-reactive, water-soluble oxygenate modifier such as analcohol with at least three carbons and preferably tert-butyl alcohol(TBA) and/or sec-butyl alcohol (SBA) is also added to theoligomerization reactor to attenuate the resin catalyst but not toparticipate in the reaction. TBA is also generated in the reaction zonewhen isobutene reacts with water over a resin catalyst. Similarly, SBAis generated from a reaction of water and normal butene. Additionally,other alcohols will form when other olefins, such as C₃ and C₅ olefinsin the feed encounter water in the presence of the resin catalyst.Moreover, olefins and alcoholic modifier react over resin catalyst togenerate ethers.

Oligomerization reaction zones in general are maintained at conditionsthat may vary widely. The temperature of the oligomerization reactionzone in which a resin catalyst is used is typically 0° to 250° C. (32°to 482° F.) and preferably 40° to 150° C. (104° to 302° F.). Pressuresin the oligomerization zone using the resin catalyst will be sufficientto maintain the liquid phase, typically 345 to 3447 kPa (50 to 500psig), and preferably 1380 to 2413 kPa (200 to 350 psig).Oligomerization conditions may also include a liquid hourly spacevelocity (LHSV) of 0.5 to 8 hr⁻¹ with 1 to 6 hr⁻¹ being preferred.

The water wash or extractor column operates at temperatures of 24° to52° C. (75° to 150° F.) and pressures of 552 to 827 kPa (80 to 120psia). The mass ratio of water to hydrocarbon is between 0.25 and 1.0and preferably between 0.6 and 0.9. Water is delivered to the columnnear the top and hydrocarbon and modifier feed are delivered near thebottom of the column. Hydrocarbons including ethers leave the top of thecolumn, substantially free of alcohol modifier and an extract stream ofwater and alcohol modifier leave the bottom of the column. In additionto TBA and SBA, substantially all of the alcohols formed in the reactionzone will exit in the extract stream. The water wash column preferablyhas a series of trays for enhancing the contact between the water andthe modifier containing hydrocarbon stream.

A debutanizing distillation fractionation column usually runs atpressures of between 413 and 1034 kPa (60 and 150 psig) and preferablybetween 517 and 827 kPa (75 and 120 psig). To make the separationbetween C₄ and C₅ hydrocarbons at those pressures, the bottomstemperature will have to be around 149° to 204° C. (300° to 400° F.) andthe overhead temperature will have to be around 38° to 66° C. (100° and150° F.) to obtain the appropriate separation.

Any suitable reflux ratio can be employed in the distillation column.The reflux ratio is the weight ratio of the portion of condensed vaporwhich is returned to the distillation column to the portion of condensedvapor which is withdrawn as distillate product. Generally, the refluxratio is in the range of from about 0.1:1 to about 2:1, and preferablyin the range of from about 0.5:1 to about 1.3:1.

Any suitable feed entry location to the fractionation column can beselected. Generally, the feed entry location is in the range of fromabout 2 to about 70 percent of the total height of the column, measuredupward from the bottom of the column. Preferably, in the context of thepresent invention, the feed entry location is in the range of from about20 to about 60 percent and more preferably in the range of from about 25to about 50 percent of the total column height.

The overhead distillate product withdrawn from the top of the columngenerally contains a larger volume percentage of the light hydrocarbonsthan the feed and a smaller volume percentage of the heavy hydrocarbonsthan the feed. The overhead product will predominately compriseunreacted light olefins, e.g., C₄ olefins. A portion of the overheadproduct may be cooled and recycled to the higher portion of the column.Generally, the bottoms product contains a larger volume percentage ofheavy components than the feed, and less of the light components thanthe feed. A portion of the bottoms product may be reheated and recycledto the lower portion of the column. Any suitable total column height andcolumn diameter and number of trays in the distillation column may beemployed. The exact dimensions and column designs depend on the scale ofthe operation, the exact feed composition, the desired recovery anddegree of purity of the product, and the like, and can be determined bythose having ordinary skill in the art.

Water washing the entire effluent from the oligomerization reactor willrequire feeding substantially more water to the water wash column.Hence, it may be desirable to arrange the piping of the apparatus toaccommodate selectively sending the oligomerization effluent either tothe water wash column directly or to the fractionation column directly.In the latter case, the bottoms product would then be sent to the waterwash column if circumstances would permit only water washing the bottomsproduct.

The invention is disclosed with reference to the FIGURE which shows anoligomerization scheme which uses a resin catalyst in theoligomerization reactor. However, other oligomerization reactionprocesses can be used in accordance with the present invention.

Feed comprising a C₄ hydrocarbon stream from an FCC debutanizerfractionation column that may have previously been water washed toremove nitriles and trace amines and statically mixed, which processesare both not shown, is brought into the process via a feed line 10. TheC₄ hydrocarbon stream typically predominantly includes mixed butenes,mixed butanes and also may include lower alkanes and olefins and C₅hydrocarbons. A modifier stream comprising an oxygenate such as analcohol and preferably tert-butyl alcohol (TBA) and/or sec-butyl alcohol(SBA) in an azeotropic mixture with water are added to the feed line 10via a modifier line 12 from a stripper column 14. The alcohol and waterfrom the modifier line 12 are combined with the feed from the feed line10 to form a combined line 16. The combined line 16 is heated by aheater 18 and enters an oligomerization reactor 20. Alternatively, themodifier line 12 and the feed line 10 enter the oligomerization reactor20 separately. In the oligomerization reactor 20, the feed contacts asolid acid catalyst, preferably a resin catalyst under oligomerizationconditions. The light olefins in the feed which are preferablypredominantly C₄ olefins oligomerize to heavy oligomers which arepreferably predominantly C₈ olefins. The oligomerization effluentcomprising unreacted light olefins, heavy product oligomers, alcoholmodifier and water is carried via a line 22 through a cooler 23 to awater wash column 24.

In the water wash column 24, a liquid-liquid extraction occurs in whichwater extracts the water-soluble, alcoholic, modifier from thewater-insoluble hydrocarbons comprising ethers, unreacted light olefinsand heavy product oligomers. Water is added near the top of the waterwash column 24 from a water line 26. The water/modifier stream exits thebottom of the water wash column 24 via an extract line 28 whereas thehydrocarbons go out the overhead of the water wash column 24 via ahydrocarbon line 30. The extract line 28 carries the water/modifiermixture to a heat exchanger 32 which exchanges heat with the water line26 to heat the water line 26. The cooled extract line 28 then enters thestripper column 14. In the stripper column 14, the water is separatedfrom the alcohol modifier. Water exits the bottom of the stripper column14 through a water line 34 which splits into two portions. A firstportion is recycled back to the water wash column 24 by the water line26. The second portion is heated in a reboiler 40 and returned to thestripper column 14 through a return line 41. The water/alcohol azeotropeis brought out the overhead via an overhead line 36. A portion of thewater/alcohol stream in the overhead line 36 is recycled back by thealcohol modifier line 12 to the influent for the oligomerization reactor20. Another portion of the stream in the overhead line 36 is transportedto further processing by a line 38.

The hydrocarbon stream in the hydrocarbon line 30 from the water washcolumn 24 is heated indirectly by an oligomer product line 44 in a heatexchanger 42. The heated hydrocarbons in the hydrocarbon line 30 arethen delivered to a debutanizer fractionation column 46. Thehydrocarbons in the hydrocarbon line 30 contain no more than traceamounts of alcohol modifier. The split in the fractionation column 46 iseffected between the C₄ and the C₅ hydrocarbons. However, as much C₅hydrocarbons can be sent to the overhead as necessary to lower the vaporpressure of the bottoms without concern that too much alcohol modifierwill be present in the overhead stream. An overhead line 48 comprisingpredominantly C₄ hydrocarbons, very little C₅ hydrocarbons and no morethan trace quantities of alcohol is split into two portions. A firstportion in a reflux line 50 is cooled in a condenser 52 and refluxedback to the fractionation column 46. The other portion of the overheadline 48 is carried to further processing by a line 54 which may includeeither a direct alkylation unit or preparation for entry into adehydrogenation unit for further indirect alkylation processing. Abottoms stream comprising C₅+ hydrocarbons, a predominant amount of C₈olefin oligomers and substantially all of the ethers in a bottoms line56 is split into a first portion which is carried by the oligomerproduct line 44 for indirect cooling by heat exchange with thehydrocarbon line 30 in the heat exchanger 42. The cooled oligomerproduct in the oligomer product line 44 then proceeds to furtherprocessing or storage. A second portion of the bottoms line 56 iscarried by a reboil line 58 to a reboiler 60 where it is heated and isrecirculated back to the fractionation column 46.

EXAMPLE

We compared the process of directly fractionating the effluent from anoligomerization zone to water washing the effluent from anoligomerization zone before fractionation by simulation. Theoligomerization zone predominantly dimerizes C₄ olefins to C₈ olefinsover a resin catalyst modified by TBA and SBA. In the fractionationcolumn, the primary split is between C₄ and lighter hydrocarbons and C₅and heavier hydrocarbons. At the operating specification that requiresthe bottoms oligomerization product to have a vapor pressure of 41 kPa(6 psi) or less, the split between C₄− hydrocarbons and C₅+ hydrocarbonswould require at least 40% of the C₅ hydrocarbons to be distilled to theoverhead. Unfortunately, under these conditions, the concentration ofTBA/SBA modifier that is carried with the C₅ hydrocarbons to theoverhead stream exceeds 1,000 ppm which is much higher than the typical5 ppm alcohol limits required of the overhead stream. In fact, to meet a5 ppm alcohol limits for the overhead, the Reid Vapor Pressure in thebottoms would have to be as high as 255 kPa (37 psi). Accordingly, toreduce the TBA/SBA modifier in the overhead stream to be within limitsand to keep the Reid Vapor Pressure of the bottoms product within theReid Vapor Pressure limits, both the overhead stream and the bottomsstream would need to be water washed.

According to the present invention, the effluent from theoligomerization reactor is water washed to remove the TBA/SBA modifierfrom the oligomerization effluent stream and then the hydrocarboneffluent stream free of TBA/SBA modifier is sent to a debutanizerdistillation column. The water wash column runs with a ratio of water tohydrocarbon of 0.75, a water wash inlet temperature of 38° C. (100° F.),a hydrocarbon feed temperature of 58° C. (136° F.) and a column pressureof about 690 kPa (100 psi). The water extracts practically all of theTBA/SBA modifier from the hydrocarbon feed. Hence, the hydrocarbon feedis then run to a debutanizer distillation column. To meet the Reid vaporpressure operating specification of 41 kPa (6 psi) in theoligomerization bottoms product of the debutanizing distillation column,over 40% of the C₅ hydrocarbons must be sent to the overhead. However,the concentration of alcoholic modifier in the overhead stream is lessthan 0.4 ppm. Hence, placing the water wash column before thefractionation column allows the vapor pressure specification in thebottom product and the alcohol specification in the overhead stream tobe met.

1. An apparatus for oligomerizing light olefins and recovering heavyolefins comprising: an oligomerization reactor vessel including a feedinlet for delivering light olefins to the reactor vessel and a productoutlet for removing light olefins and heavy olefins from the reactorvessel; a water wash column including a hydrocarbon inlet incommunication with said product outlet of said reactor vessel, a waterinlet near a top thereof, a water outlet near a bottom thereof and ahydrocarbon outlet near a top thereof; and a separation column includinga hydrocarbon feed inlet in communication with said hydrocarbon outletof said water wash column, a light olefin outlet near a top thereof anda heavy olefin outlet near a bottom thereof.
 2. The apparatus of claim 1further including a stripper column having an inlet in communicationwith said water outlet of said water wash column.
 3. The apparatus ofclaim 2 wherein said inlet to said stripper column is near a top of saidstripper column.
 4. The apparatus of claim 2 wherein said stripperincludes a water outlet in communication with said water inlet of saidwater wash column for recycling water from said stripper column to saidwater wash column.
 5. The apparatus of claim 2 wherein said stripperincludes a modifier outlet in communication with said oligomerizationreactor vessel.
 6. The apparatus of claim 5 wherein said modifier outletis in communication with said feed inlet to said oligomerization reactorvessel.
 7. An apparatus for oligomerizing light olefins and recoveringheavy olefins comprising: an oligomerization reactor vessel including afeed inlet for delivering light olefins to the reactor vessel and aproduct outlet for removing light olefins and heavy olefins from thereactor vessel; a water wash column including a hydrocarbon inlet incommunication with said product outlet of said reactor vessel, a waterinlet near a top thereof, a water outlet near a bottom thereof and ahydrocarbon outlet near a top thereof; a separation column including ahydrocarbon feed inlet in communication with said hydrocarbon outlet ofsaid water wash column, a light olefin outlet near a top thereof and aheavy olefin outlet near a bottom thereof; and a stripper column havingan inlet in communication with said water outlet of said water washcolumn, said stripper column having a water outlet in communication withsaid water inlet of said water wash column for recycling water from saidstripper column to said water wash column.
 8. The apparatus of claim 7wherein said inlet to said stripper column is near a top of saidstripper column.
 9. The apparatus of claim 7 wherein said stripperincludes a modifier outlet in communication with said oligomerizationreactor vessel.
 10. The apparatus of claim 9 wherein said modifieroutlet is in communication with said feed inlet to said oligomerizationreactor vessel.